This invention relates to an airborne radar system, and in particular, to avionics and airborne collision avoidance radar systems, that utilizes Automatic Dependent Surveillance-Broadcast (ADS-B) transponders.
Due to increasing air traffic and capacity limitations of conventional or primary radar systems, there is a need for the next generation surveillance systems to insure aircraft safe separation. Primary surveillance radar systems operate by sending out microwave energy that is reflected back by an aircraft's metallic surfaces. The reflected signal is received back at the radar transmission site and displayed as location information. The primary surveillance radar systems may be either land-based or airborne. For land-based systems, the received information may be used by an air traffic controller to separate air traffic. However, land-based primary surveillance radar has limitations at low altitudes, does not work on the ground, and may also be influenced by atmospheric and weather conditions. Airborne primary surveillance radar is expensive and typically limited to large commercial or military aircraft. Accordingly, there has been a drive to establish secondary surveillance radar systems to supplement the primary surveillance radar.
One type of secondary surveillance radar system is known as Traffic Alert and Collision Avoidance System (TCAS). The TCAS system works with cooperative targets equipped with TCAS avionics. When one TCAS-equipped aircraft approaches a second aircraft, the first aircraft determines the location of the second aircraft by transmitting an interrogation signal at regular intervals. If the second aircraft is equipped with a compatible transponder (one such type of transponder is known as a Mode A transponder), the transponder receives the interrogation signal and responds with a signal, which allows the first aircraft to calculate the range and bearing of the second aircraft based upon direction and relative strength of the signal. Certain types of transponders also include altitude information in the response. For receiving and transmitting signals with a TCAS system, it is known to use both top and bottom L-band circular antennas.
The second aircraft may be equipped with a different type of transponder (Mode S) that may alter the method of interrogation between the aircrafts. For instance, the second aircraft may automatically transmit a message containing the address of the transmitting aircraft at set intervals, such as once a second. Then, the first aircraft can send a direct interrogation to the second aircraft, based upon the known address of the second aircraft. The second aircraft responds with a signal that includes altitude information. To minimize interference with other aircraft, the rate at which a Mode S transponder is interrogated may depend upon the range and closing speed of the two aircraft.
One example of a TCAS system is disclosed in U.S. Pat. No. 5,636,123 to Rich et aL. The system in Rich describes a TCAS system, wherein air space is divided into a grid of volume elements, and each volume element is assigned a unique pseudo-noise code. A collision avoidance signal is generated by a transmitter on an aircraft, wherein the transmission includes an appropriate pseudo-noise code for the volume element in which the aircraft is positioned.
Another TCAS system is disclosed in U.S. Pat. No. 6,459,411 B2 to Frazier et al. The system is for aircraft flying in formation and is based upon receiving and processing Mode-S transponder messages without the TCAS computer having to interrogate the transponders of the other aircraft flying in formation. The system is designed for the TCAS computer to receive and process data without having to interrogate transponders to allow safe separation between 2 to 250 aircraft flying in formation.
Yet another TCAS system is disclosed in U.S. Pat. No. 6,278,396 B1 to Tran. The TCAS system in Tran includes a module designed to include multiple phases of a tactical mission including formation fall-in, arming formation flight, engaging formation flight following, formation breakaway and air-refueling sequencing. The tactical module divides the air space around the aircraft into advisory, caution, and warning zones and provides displays and alerts in accordance with the appropriate zone.
Another TCAS system is disclosed in U.S. Pat. No. 7,006,032 B2 to King et al. The TCAS system disclosed by King utilizes a Mode-S transponder in an integrated L-band traffic surveillance apparatus embedded in a signal unit replaceable processor.
ADS-B is a surveillance system used by aircraft to inform other aircraft and ground stations about location, speed, and intention. ADS-B is becoming one of the most important technologies for air transportation. ADS-B-equipped aircraft broadcasts its position in space with a digital data link and can include other data, such as air speed, altitude and air-maneuver information. ADS-B receivers can be integrated into the air traffic control system or installed aboard other aircraft to provide an accurate depiction of real-time aviation traffic, both in the air and on the ground. ADS-B relies on satellite-based Global Positioning System (GPS) to determine an aircraft's precise location in space. The system converts the position into a digital code, which is combined with other information that can include the type of aircraft, speed, flight number, and maneuvering information. The digital code containing the information is updated at regular intervals, such as several times a second, and broadcasts from the aircraft on a discreet data link frequency. Other aircraft and ground stations up to approximately 150 miles can receive the data link broadcast and display information on a computer display.
One system that utilizes an ADS-B receiver is disclosed in U.S. Pat. No. 6,064,335 to Eschenbach. The system disclosed in Eschenbach includes a GPS receiver and an ADS-B receiver for receiving an ADS-B signal from an aircraft. The system also includes a visor for displaying a three-dimensional bearing of the aircraft based upon a GPS signal and an ADS-B signal.
Another system that utilizes ADS-B signals is disclosed in U.S. Patent Application Publication 2003/0154018 A1 to Lai. The system disclosed by Lai is designed to correlate between targets from two different target reporting sources, such as ADS-B and an on-the-ground traffic information system to provide information to nearby aircraft that are suitably equipped and within range. The system uses a fuzzy logic to produce a continuous confidence level on a component comparison between the two different target-reporting systems.
U.S. Pat. No. 6,789,016 B2 to Bayh et al. discloses an integrated airborne transponder collision avoidance system. The system disclosed by Bayh is designed to allow an ADS-B system to interoperate with commercial TCAS-equipped aircraft without significantly enlarging the volume of ADS-B equipment and without requiring additional antennas to be mounted on top and underneath the fuselage.
A limitation of ADS-B technology is that it relies on GPS, which can be jammed or compromised and can only display cooperative targets, that is, aircraft that transmit an ADS-B signal. TCAS systems have similar limitations and primary radar sensors are necessary to provide situational awareness or collision alerts for non-cooperative targets or hazards, such as birds, air turbulence, unmanned aerial vehicles, military aircraft, and general aviation aircraft without Mode-C or Mode-S transponders. However, the weight and cost of providing airborne primary radar makes this technology economically viable for only large and/or military aircraft. In addition, TCAS uses analog, beam-forming techniques and amplitude-comparison on a pulse for direction finding. Digital beam forming and spectrum estimation techniques could be used to provide an avionics improvement.
Therefore, it is an object of the present invention to provide a secondary surveillance system radar utilizing ADS-B transponders that can provide collision-avoidance information for non-cooperative targets. An additional object of the invention is that the avionics utilize a digital-beam-forming modulation of the ADS-B signal. A further object of the invention is that the system may utilize conventional top and bottom circular array antennas as are currently used on TCAS-equipped aircraft. A further object of the invention is to provide a digital matched filter and digital signal processor to analyze signals received by the antennas. Also, it would be desirable if the system could estimate the angle-of-arrival of reflected ADS-B signals so that both the azimuth and elevation of the signal may be determined.